Wafer coating method for flip chips

ABSTRACT

A method for applying an underfill and edge coating to a flip chip is described. The method includes the steps of adhering a bumped wafer to an expandable carrier substrate, sawing the wafer to form individual chips, stretching the carrier substrate in a bidirectional manner to form channels between each of the individual chips, applying an underfill material to the bumped surfaces of the chips and around the edges of the chips, cutting the underfill material in the channels between the chips and removing the individual, underfill coated chips from the carrier.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/067,381, entitled “Flip Chip With Integrated Flux and Underfill”filed Apr. 27, 1998, and a continuation-in-part of U.S. application Ser.No. 09/266,166, entitled “Flip Chip With Integrated Mask and Underfill”filed Mar. 10, 1999 now U.S. Pat. No. 6,228,678.

FIELD OF THE INVENTION

The present invention relates generally for methods of providingunderfill materials on flip chips. More particularly, the presentinvention relates to a method in which a wafer is provided with solderbumps and diced prior to the application of a polymeric underfillmaterial.

BACKGROUND OF THE INVENTION

Flip chip technology, although several decades old in the ceramic modulearea, has recently been applied to organic substrates. Flip chipmethodologies, first employed over 30 years ago, involve directlyattaching Integrated Circuits (ICs) to circuit boards by means of ajoint instead of a wire. This method is also called Direct Chip Attach(DCA). Several mainframe computer makers and other manufacturers ofelectronic components, have used this flip chip technology to producevery efficient, high-density modules. The direct attachment of the ICsto circuit boards eliminates the IC component package that can occupy upto 50 times more space than the IC chip itself.

Many companies have used flip chips exclusively on ceramic circuits thathave a low Coefficient of Thermal Expansion (CTE). In theseapplications, the chip and circuit have a relatively closethermomechanical match. As a result, upon heating, the IC and thesubstrate upon which it is mounted undergo similar thermal expansionswith the result being that physical stresses between the IC and thesubstrate are minimized or maintained within acceptable limits. However,when flip chip technology is applied to the more common and considerablylower cost organic printed circuit boards, a large thermal mismatchresults. Typical organic printed circuit boards (PCBs) have a CTE of 15to 25 parts per million per ° C. (ppm/° C.) compared to only 2-3 ppm/°C. for silicon ICs. The results are that excessive stress is generatedduring thermal cycling of flip chips that have been attached to PCBs.These forces can be great enough to destroy the joints that connect thechip to the circuit.

One common solution to the flip chip thermal mismatch problem has beenknown for some time. Specifically, a liquid polymerizable material,called an “underfill”, is flowed under the flip chip. Once the underfillhas completely filled the small gap that exists between the bottom ofthe flip chip and the substrate, the material is hardened bypolymerization. The hardened, polymerized underfill locks the flip chipand circuit board together so that there is little if any differentialmovement. As a result, the very rigid flip chip is mechanically coupledto the substrate by means of the underfill. Thus, the substrateexpansion, at least at the interface, is restrained. This restraintcauses the substrate adjacent to the flip chip to physically behave in amainer that is similar to that of the flip chip. By controllingexcessive stresses that would otherwise form in the joints between thechip and PCB, a reliable assembly can be fabricated.

While the use of underfill has solved the thermal mismatch problem forlow cost flip chips on organic substrates, it has given rise to a seriesof significant manufacturing problems. First, the prepolymerized liquidunderfill must be applied off-line as a secondary process with specialequipment. Typically, the underfill is applied to one, two or threeedges of the assembled flip chip and allowed to flow all the way underthe mounted chip. Once the material has flowed to opposite edges and allair has been displaced from under the chip, additional underfill isdispensed to those outer edges to form a fillet so that that all fouredges are symmetrical. The fillet increases reliability and is generallypreferred even though it requires additional manufacturing time. Nextthe assembly is baked in an oven to polymerize and harden the underfill,again adding time to the process. This baking process can take severalhours, although, new processes, such as microwave curing, may reduce thetime substantially. However, the added equipment and its maintenanceadds significantly to manufacturing costs. Thus, while the use ofunderfill helps to alleviate the thermal mismatch problem and provides acommercial solution, the electronic device manufacturing industry seeksmore efficient manufacturing methods with lower associated costs. Infact, many in the industry believe that the present burden imposed byunderfill processing may inhibit industry wide use of flip chiptechnology.

Recently, advances have been made which improve and streamline theunderfill process. One method that has shown some commercial interestinvolves dispensing underfill before assembling the flip chip to theboard. The method requires that the underfill permit solder joints toform. Soldering of flip chips to circuits is generally accomplished byapplying flux to the solder bumps on the flip chip or to the circuitpads on the circuit. An underfill that is pre-dispensed before solderingshould contain flux or have properties that facilitate solder jointformation. Since the pads on circuit often oxidize and since tin/leadsolder bumps on flip chips are always oxidized, the flux must bedesigned to reduce these oxide coatings and facilitate solder jointformation. These so called, pre-dispensed underfills, have thereforebeen designed to include flux chemistry. While simplifying the flip chipprocess, they still require extra steps and cannot be run on a standardsurface mount assembly line.

In spite of the numerous advantages provided by flip chip technology, aneed still exists for flip chip methods which simplify the applicationof underfill. A need also exists for process methodologies which reducethe number of process steps required with underfill application.

SUMMARY OF THE INVENTION

The present invention relates to a method for applying underfill to flipchips prior to assembling such chips on printed circuit boards. Theinvention is characterized in that it simplifies the process of applyingthe underfill, permits conventional sawing methods to be used insingulating the wafer and provides underfill not only on the undersideof each chip but along each edge of the chip as well.

More particularly, the present invention relates to a process whichincludes the steps of providing an expandable adhesive substrate havingadhered thereto a wafer having solder bumps, cutting or sawing the waferto define one or more individual integrated circuit chips, stretchingthe adhesive substrate in a manner which separates each individualintegrated circuit from those adjacent to it, and providing an underfillmaterial on the bumped surface and along each edge of each integratedcircuit. The resulting underfill-coated chips can then be removed fromthe adhesive substrate and individually packaged for transport andstorage until they are applied to a printed circuit.

Once positioned on a printed circuit board, the assembly is transportedthrough a reflow oven or other means in which to heat the solder andpolymerize the underfill, thereby effectively mounting the chips inplace. The resulting chips are soldered into position on the printedcircuit board, and include a complete underfill and surroundingpolymeric fillet. Of course, if the underfill material is aprepolymerized, thermoplastic material, the heating step serves to meltthe underfill, allowing it to flow and bond between the chip and theprinted circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a wafer adhered to a temporary,expandable carrier substrate.

FIG. 2 is a schematic representation of a cut wafer adhered to atemporary, expandable carrier substrate.

FIG. 3 is a schematic representation of a cut wafer adhered to atemporary, expandable carrier substrate following stretching of thesubstrate.

FIG. 4 is a schematic representation of the wafer of FIG. 3 after anunderfill coating has been applied to the wafer surface.

FIG. 5 is a schematic representation of a bumped flip chip having anunderfill material covering its underside and edges.

DETAILED DESCRIPTION OF THE INVENTION

This invention involves a novel coating method for applying an underfillalone, flux in combination with an underfill, or combined flux-underfillliquids to a semiconductor wafer that has been pre-cut, or sawn, wherethe singulated chips are temporarily held in position by anadhesive-coated film tape. The particular flux and underfill materialsare described in co-pending U.S. patent application Ser. No. 09/067,381,entitled “Flip Chip With Integrated Flux and Underfill”, and inco-pending U.S. patent application Ser. No. 09/266,166, entitled “FlipChip With Integrated Mask and Underfill”. The teachings of theseco-pending patent applications are incorporated herein by reference.

Several benefits are provided by the present invention. First, bysingulating the semiconductor wafer prior to applying the underfillmaterial, the difficult step of sawing the wafer and the hardenedintegral underfill material simultaneously is avoided. Second, theinvention eliminates difficulties associated with visually locating thecutting lines, (called “streets”), on the wafer since sawing is carriedout before application of underfill material. Third, the presentinvention yields chips having underfill coating each of their fouredges. This edge coating provides the additional underfill materialneeded to form edge fillets during the solder reflow process. As notedabove, fillets have been shown to increase the performance of underfill.

The invention is a multi-step process which can be described as follows.An uncut wafer having solder bumps provided on one surface is adhered toa temporary, expandable adhesive-coated carrier film. One preferred filmof this type is commonly referred to as “Nitto Tape” (commerciallyavailable from Nitto-Denko, Ltd.). Once the wafer has been adhered tothe carrier film, it is cut into individual chips in a process referredto as “singulation”. Cutting is commonly achieved using a precision sawwhich is designed to make a narrow cut having clean edges. Additionally,the cutting process must be one which does not cut through the carriersubstrate. Rather, the singulation is carried out in a manner thatdivides the wafer into individual chips, while leaving the carrier filmintact. Following singulation, the tape is stretched or expand in acontrolled, multidirectional manner. In so doing, the individual chips,while remaining adhered to the carrier substrate, are separated fromeach other, and spaces or channels are formed between them. To thispoint, the process described above is substantially the same as thosecurrently known in the art.

The inventive portion of the process occurs following stretching of thecarrier substrate and the related separation of the individual chips.Specifically, the separated chips, while still adhered to the carrier,are coated with the underfill material. In embodiments in which flux isnecessary and is applied separately, the flux can be applied at thisstage as well. Alternatively, in embodiments employing a flux-containingunderfill or an underfill having fluxing properties, just the underfillmaterial is applied to the wafer. The underfill is applied in a mannersuch that it covers the bumped surface to an appropriate depth(depending upon its fluxing characteristics) and also in a manner suchthat it surrounds the edges of each singulated chip. As discussed above,this edge coating provides desirable properties to the resulting filletupon curing of the underfill material.

Once the underfill material has been applied to the bumped surface ofeach chip and its edges, it is caused or allowed to dry, and then thedried coating that lies in the spaces or channels between each chip iscut to thereby separate each coated chip for packaging and subsequentprocessing.

As noted previously, the steps of mounting a wafer on an expandable,adhesive carrier substrate, cutting the wafer into singulated chips, andthen expanding the adhesive carrier substrate to isolate the individualchips from one another is well-known in the art. These steps are doneroutinely in the semiconductor and packaging fields. Equipment forcarrying out these steps is available from several suppliers (such asHover-Davis). This equipment typically is designed to stretch thecarrier tape equally so that the space between the chips isapproximately equal. This is normally done to permit easy chip removalfrom the tape without damaging chip edges as the chips are “picked” offthe carrier. Unlike the prior art, one consideration of the presentinvention is to provide a controlled, preferably uniform, spacing thatwill become filled with liquid underfill material during the coatingprocess. One method for adjusting the amount of underfill materialapplied to each chip is to alter the spacing between each chip. Thisadjustment is useful because the thickness of the underfill coatingapplied to the wafer will vary with the type of flip chips and theirbump height.

As noted above, once the adhered wafer has been cut into individualchips and the spacing between those chips has been achieved to thedesired value via stretching of the carrier, the underfill material canbe applied. The incorporated applications describe two differentflux-underfill systems, each of which employs a different set of coatingsteps when used in connection with the process of the present invention.

The process described above may be best understood with reference to theFigures. In FIG. 1, the adhered workpiece 10 comprises a bumped wafer 12adhered to a surface of a temporary, expandable carrier substrate. Asshown in FIG. 2, the wafer 12, while still adhered to the carriersubstrate 14 has been sawed or cut along several pathways 16 to define aplurality of individual bumped chips 18. Following the cutting, thesubstrate 14 is stretched, as shown in FIG. 3, to isolate or singulatethe individual flip chips 18 by forming spaces or channels 20 betweenthem. In FIG. 4, the workpiece 10 has then been coated with theunderfill material 22 which covers both the singulated chips 18 as wellas the channels 20 between such chips. The underfill material 22occupying the channels 20 is then cut and the individual underfillcoated chips can be removed and packaged.

One such chip is shown schematically in FIG. 5. In FIG. 5, theunderfilled, bumped chip 24 includes a singulated integrated circuitchip 18 having solder bumps 26 on its underside. An underfill coatinghas been provided in a manner such that it covers the underside of thechip (22 a) and also extends a short distance beyond the edges of thechip (22 b) in order to effectively provide a fillet having underfillwhich entirely surrounds the edge of the chip as well as coating theunderside of the chip.

A two-component system having a separate flux and underfill composition,has been described in the previously incorporated application U.S. Ser.No. 09/067,381. In the two-component flux and underfill compositionembodiment of the invention, the flux may be applied before theunderfill by a dipping process. While dipping can be prior to sawing thebumped wafer, a preferred method is to apply the flux after the waferhas been sawed in order to avoid contamination of the flux by particlesof silicon which may result from the cuts. The flux can be appliedbefore or after the carrier substrate is stretched to isolate theindividual chips. However, for large wafers, it is desirable to applyflux to the bumps before sawing so that high planarity is maintained.Furthermore, to be effective, flux only needs to be applied to thebumps, rather than to the entire underside of the flip chip, and thiscan be done using rolling or by dipping methods. The flux is allowed todry on or entirely around the solder bumps.

The workpiece, comprising the sawed and flux coated wafer adhered to thecarrier substrate, is now expanded or stretched via stretching of thecarrier substrate to the give the desired spacing between chips. Thestretching is preferably bidirectional to achieve the appropriatespacing between each chip. In some embodiments, this stretching may havebeen performed prior to the application of the flux, while in otherembodiments, the stretching may occur after the application of the flux.

It should be noted that the flux described preferably has a low surfaceenergy. As such, the flux resists over-coating by the underfill. Ofcourse, as an alternative, the flux can be applied after the underfillhas been coated and dried. It is noted that when employing an embodimentin which the flux and the underfill are separate compositions, themethod used to provide the flux coating should only apply flux to thebumps, most preferably to the top surface of the bumps, since flux isnot needed on the sides of the chip.

The underfill liquid is now ready to be applied. It is preferred thatthe coating methods produce a symmetrical deposition. Such methodsinclude, but are not limited to, stencil printing, curtain coating andspraying. Spin coating, although possible, is generally not recommendedbecause “shadowing” of cuts that are perpendicular to spin-induced flowdirection may be deficient in material and the final coating may not beof uniform thickness. Once the underfill material has been applied, itis heated in order to dry the underfill coating. Heating is generallyconducted for a period of time sufficient to bake out the solventcontained in the underfill material.

Dried coating contained in the channels between the singulated chips isthen cut in order to separate the chips. This separation can be donewith a wafer saw since the hardened underfills generally comprise highmelting engineering thermoplastics with good sawing characteristics.Laser cutting can also be used, as can a numerically controlled knifecutting (NC knife). It is preferred that the carrier substrate not becut through since chip removal is much easier if the tape remains as asingle film.

A one-component coating system, having a combined flux and underfillcomposition, is described in the previously incorporated applicationU.S. Ser. No. 09/266,166. The same underfill coating methods as justdescribed for the underfill portion of the two-component composition canbe used for the one-component flux-underfill system. The cutting methodfor the dried material is also the same.

Following application of the coating to the underside of the chip andaround its edges, the chips can be assembled to printed circuit boardsusing reflow soldering methods. The specifics of such assembly have beendescribed in the previously incorporated applications. Of course, priorto use, the chips must be removed from the carrier substrate, and suchchips are typically then stored in waffle packs or other standardpresentation holders. The same equipment manufacturers that sellexpanding (i.e., stretching) equipment offer machines that can pick upand place chips from the substrate. The common mechanism is to pull onthe chip with a vacuum tool while simultaneously pushing against theback of the substrate with an “ejector” pin.

Upon exposing the inventive chips to reflow soldering, they will becomesoldered to a printed circuit board while simultaneously being providedwith an underfill material positioned both between the chip and theprinted circuit board, as well as around the edges of the chip. The edgecoating is desirable because during the reflow heating, the underfill,being thermoplastic, softens or melts to form a fillet around theattached flip chip. The fillet improves bond strength and reducesstresses, particularly thermally-induced stresses, at the edge of thechip.

EQUIVALENTS

From the foregoing detailed description of the specific embodiments ofthe invention, it should be apparent that a novel heat transfer labeland heat transfer labeling system has been described. Althoughparticular embodiments have been disclosed herein in detail, this hasbeen done by way of example for purposes of illustration only, and isnot intended to be limiting with respect to the scope of the appendedclaims which follow. In particular, it is contemplated by the inventorthat various substitutions, alterations, and modifications may be madeto the invention without departing from the spirit and scope of theinvention as defined by the claims.

What is claimed is:
 1. A method for providing an underfill material onan integrated circuit chip which comprises the steps of: a) providing awafer containing at least one integrated circuit chip, the integratedcircuit chip having at least one solder bump provided on an exposedsurface thereof; b) affixing the wafer to an expandable carriersubstrate in a manner such that the bumped surface of the integratedcircuit chip is exposed; c) cutting the wafer in a manner which definesedges on said at least one integrated circuit chip, the cutting being ofa depth which does not cut the carrier substrate; d) expanding thecarrier substrate in a manner which singulates said at least oneintegrated circuit chip, the singulation forming channels which surroundthe integrated circuit chip; e) providing an underfill material on thebumped surface of the integrated circuit chip and in the channelsdefined by the edges of the integrated circuit chip; and f) cutting theunderfill contained in the channels to thereby define at least onebumped integrated circuit chip having an underfill material provided onits bumped surface and surrounding its edges.
 2. The method of claim 1,wherein at least the said at least one solder bump is provided with aflux material.
 3. The method of claim 2, wherein the flux is provided bythe underfill material.
 4. The method of claim 2, wherein the flux isprovided in an additional process step.
 5. The method of claim 4,wherein the flux is provided before the carrier substrate is expanded.6. The method of claim 1 wherein the expansion of the carrier substrateis a bidirectional expansion.
 7. The method of claim 1, wherein theunderfill material is provided to a depth which allows the at least onesolder bump to be exposed.
 8. The method of claim 1, wherein theunderfill material covers the at least one solder bump.
 9. A method forproviding an underfill material on an integrated circuit chip whichcomprises the steps of: a) providing at least one integrated circuitchip adhered to a temporary expandable carrier substrate havingsingulated channels formed thereon, the integrated circuit chip havingan exposed surface and a plurality of edges defined by said singulatedchannels, the exposed surface having at least one solder bump thereon;and b) providing an underfill material on the bumped surface of theintegrated circuit chip and on its edges.
 10. The method of claim 9,wherein at least the said at least one solder bump is provided with aflux material.
 11. The method of claim 10, wherein the flux is providedby the underfill material.
 12. The method of claim 10, wherein the fluxis provided in an additional process step.
 13. The method of claim 9,wherein the underfill material is provided to a depth which allows theat least one solder bump to be exposed.
 14. The method of claim 9,wherein the underfill material covers the at least one solder bump.